The present invention relates to expanded polypropylene beads having a density of from 5 to 200 g/l and a bead diameter of from 1 to 10 mm based on a propylene polymer. Expanded polypropylene (EPP) beads are employed on an industrial scale for the production of foam moldings. These are predominantly employed as elastic, impact-resistant packaging materials which also have thermal insulation properties. However, their thermal conductivity is still too high for some applications.
According to the prior art, polyolefin foam beads of low thermal conductivity can be produced by employing infrared-reflecting metal pigments, such as aluminum powder (DE-A 195 05 266). Disadvantages in this method are the high price and the restricted availability of the requisite pigments.
In order to produce foam moldings from EPP beads, the latter are introduced into a mold, expanded further by various methods (see, for example, DE-A 3431245) and fused to one another by means of steam or hot air. During this operation, a pressure builds up inside the molding. In order to prevent the foam cells from bursting, the molding can, after conventional cooling, only be removed from the mold when the pressure has dropped to about 0.3 bar (above atmospheric). The throughput in molding production thus depends essentially on the pressure dissipation time.
It is thus an object of the present invention to provide EPP beads which can be converted into foam moldings having reduced thermal conductivity, and to shorten the pressure dissipation time.
We have found that these objects are achieved by EPP beads which comprise from 0.002 to 20% by weight of graphite particles in homogeneous distribution. In order to reduce the thermal conductivity, amounts of from 1 to 20% by weight are preferred, in particular from 2 to 10% by weight, for shortening the cooling time from 0.002-10% by weight, in particular from 0.005 to 5% by weight. During molding production, shrinkage occurs if the graphite content is too high, i.e. the moldings shrink after production. If this is to be prevented, the amount of graphite must be kept low. In this case, amounts of from 0.002 to 0.2% by weight are preferred.
For the purposes of the present invention, propylene polymers are homopolypropylene and random copolymers of propylene with up to 15% by weight, preferably from 0.5 to 12% by weight, of ethylene and/or a C4-C10-xcex1-olefin, preferably a copolymer of propylene with from 0.5 to 6% by weight of ethylene or 1-butene or a terpolymer of propylene, from 0.5 to 6% by weight of ethylene and from 0.5 to 6% by weight of 1-butene.
The propylene polymers are prepared by conventional processes using Ziegler-Natta catalysts and are generally obtained in the form of linear polymers. For the extrusion foaming process described below, branched propylene polymers, for example in accordance with EP-A 588 321, are also suitable. Propylene polymers prepared using metallocene catalysts, in accordance with EP-A 611 795, may also advantageously be employed. These propylene polymers are mixed with the graphite particles by conventional methods in the melt, preferably in an extruder. These graphite particles preferably have a particle size of from 1 to 200 xcexcm, in particular from 1 to 50 xcexcm. Expandable graphite can also advantageously be employed.
The EPP beads according to the invention are produced by the impregnation or foam extrusion process known per se. In the impregnation process, granulated polypropylene containing graphite particles is suspended in water together with from 1 to 40% by weight, preferably from 5 to 30% by weight, of a volatile blowing agent. Suitable blowing agents are hydrocarbons, for example butane, pentane, cyclopentane, cyclohexane or ethanol, furthermore gases, such as carbon dioxide, noble gas, air or nitrogen, and mixtures of these blowing agents. The impregnation is preferably carried out in a tubular reactor at temperatures from 100 to 160xc2x0 C. under a pressure of from 10 to 100 bar and residence times of from 30 to 180 minutes. The pressure is subsequently released, and the foam beads form.
In the extrusion process, the propylene polymer together with the graphite particles and from 2 to 30% by weight of blowing agent is melted at from 120 to 240xc2x0 C. in an extruder, during which it is dissolved. After a residence time of from 0.5 to 90 minutes, the blowing agent-containing polymer is extruded through a die with expansion and is granulated.
Besides the graphite particles, conventional additives, such as dyes, pigments, nucleating agents, stabilizers, flame retardants, lubricants and antistatics, can be added to the propylene polymer.
The EPP beads according to the invention have a density of from 5 to 200 g/l, preferably from 15 to 120 g/l, and a particle diameter of from 1 to 10 mm. They are predominantly closed-cell with a cell count of preferably from 5 to 500 cells per mm.
The EPPs can be fused by conventional methods with a short pressure dissipation time using steam or hot air to give foam moldings, which are used where, besides the elastic properties, good thermal insulation is also important. Examples which may be mentioned are refrigerator insulation and packaging of heating circuit distributors, which are mounted together with the packaging material.